#include <iostream>
using namespace std;

int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);

  int a, b, f, k;

  cin >> a >> b >> f >> k;
  if (b < f) {
    cout << -1 << endl;
    return 0;
  } else {
    int df = b - f;
    int ans = 0;

    for (int i = 1; i < k; i++) {
      if (i & 1) {
        if (df < 2 * (a - f)) {
          ans++;
          df = b;
        }
        df -= 2 * (a - f);
      } else {
        if (df < 2 * f) {
          ans++;
          df = b;
        }
        df -= 2 * f;
      }
      if (df < 0) {
        cout << -1 << endl;
        return 0;
      }
    }
    if (k & 1) {
      if (df < a - f) {
        ans++;
        df = b;
      }
      if (df < a - f) {
        ans = -1;
      }
    } else {
      if (df < f) {
        ans++;
        df = b;
      }
    }
    cout << ans << endl;
  }
  return 0;
}
